Magnetic thin films (soft magnetic thin films) having a low characteristic value of coercivity are widely used for electronic components such as magnetic heads, small transformers, meter gauges and magnetic shieldings.
Soft magnetic thin films such as cobalt based alloy films or permalloy films having an iron content of 50 to 60% by mass have high saturation magnetic flux density and are used for magnetic heads of AV. On the other hand, permalloy films having an iron content of around 20% by mass have low magnetic flux density; but due to their high initial magnetic permeability, they are used for small transformers, meter gauges, magnetic shieldings and the like.
Nickel-iron alloy films may be prepared by a method in which electroplating is carried out with a plating solution containing divalent iron ions and divalent nickel ions.
However, when a nickel-iron alloy plating solution containing divalent iron ions is left over, iron ions are oxidized to be trivalent iron ions and iron (III) hydroxide is precipitated. During plating, divalent iron ions are oxidized to trivalent iron ions at the anode side, resulting in the occurrence of the precipitation of iron (III) hydroxide. The precipitation of iron (III) hydroxide is dispersed in the plating solution and incorporated into a plating film, causing poor appearances or a decrease in the saturation magnetic flux density of the film. Accordingly, it is desirable to prevent the occurrence of the precipitation of iron (III) hydroxide.
The precipitation of iron (III) hydroxide during nickel-iron alloy electroplating may be suppressed, for example, by a method in which a compound capable of forming stable complex ions with trivalent iron ions such as a dicarboxylic acid is added to the plating solution (Patent document 1). By adding a dicarboxylic acid such as malonic acid and adjusting the pH to 1.5, trivalent iron ions are stabilized as complex ions and occurrence of the precipitation is suppressed. Although the added complexing agent such as malonic acid in this method can suppress occurrence of the precipitation, it cannot suppress the oxidation of iron ions from divalent to trivalent. As a result, a plating film having a stable composition cannot be obtained because the quantities of electricity required for deposition of the divalent and trivalent ions are different, and it is difficult to maintain the composition of iron at 18 to 22% by mass in the deposited film during plating.
It has been also known that an addition of a reducing agent can suppress the production of trivalent iron ions and allows stable continuous operation. For example, in Patent document 2, a reducing agent such as L-ascorbic acid and gallic acid is added to an iron group alloy plating solution and the pH is adjusted to 1 to 5, in order to suppress the production of trivalent iron ions. However, the occurrence of the precipitation of iron (III) hydroxide could not be sufficiently suppressed, even with the addition of a reducing agent such as L-ascorbic acid and gallic acid.
Accordingly, none of the above processes can sufficiently suppress the precipitation of iron (III) hydroxide in nickel-iron alloy electroplating solutions and there have been a difficulty in obtaining soft magnetic films.